In the conventional automobile painting processes, a metallic coating composition containing a metallic pigment is applied to coated sheets or plates having an undercoat layer and an intermediate coat layer to form an overcoat or top coat layer and then a clear coat paint is further applied onto the top coat layer by the wet-on-wet method without curing said top coat layer. The metallic base coating and clear coating are then simultaneously cured or baked. This is the so-called two coat one bake coating method (2C1B method).
From the health and environment protection view points, it is desirable that such paints be in the form of aqueous or powder coating compositions, not in the form of solvent types.
In the case of 2C1B method, it is the most practical that the metallic base coating composition is in the form of an aqueous coating composition and the clear coating composition is in the form of a powder coating composition.
In an attempt to provide aqueous metallic base coating compositions, Japanese Kokai Publication Sho-58-168664, for instance, discloses the use of aqueous dispersions of polyurethanes for the preparation of metallic base coating compositions, while Japanese Kokai Publication Hei-01-287183 discloses aqueous metallic base coating compositions containing an acrylic emulsion, a urethane emulsion and a cross linking agent.
However, such coating compositions are not yet satisfactory in the orientation of metallic pigments contained therein, hence the resulting paint films are not fully satisfactory in appearance.
The present inventors previously proposed an aqueous metallic coating composition in which a mixture composed of (a) 95 to 10% by weight (as solids) of a film-forming polymer prepared by copolymerizing amide group-containing ethylenic monomers, acidic group-containing ethylenic monomers, hydroxyl group-containing ethylenic monomers and other monomers to give a copolymer having a number average molecular weight of 6,000 to 50,000, and neutralizing at least part of the acidic groups of said copolymer, and (b) 5 to 90% by weight (as solids) of a urethane-containing aqueous dispersion prepared by dispersing, in a primary and/or secondary polyamine containing aqueous medium, hydrophilic group-containing oligomers produced by reacting a hydroxyl-terminated diol compound or compounds having a molecular weight of 100 to 5,000, a diisocyanate compound or compounds and a hydrophilic group-containing compound or compounds having at least one active hydrogen atom within the molecule together is used (Japanese Kokai Publication Hei-04-025582).
When the aqueous metallic coating composition mentioned above is used in combination with solvent type clear paints comprising acrylic resin varnishes, melamine resin varnishes and acrylic polymer micro particles incorporated therein, or solvent type clear paints comprising an acrylic resin varnish, isocyanate curing agent, high nonvolatile matter contents can be accomplished and, in addition, very excellent orientation of metallic pigments or, in other words, excellent paint film appearance, can be attained.
However, nothing is mentioned of any powder clear coating composition in Japanese Kokai Publication Hei-04-025582.
When the base coating composition is aqueous and a powder clear coating is applied without curing the base, the powder coating composition shows a higher viscosity and a greater degree of shrinkage upon cooling than the base paint film surface of the base coating composition. As a result, the paint interface is disturbed and the appearance tends to deteriorate. Attempts to develop a two coat one bake method in which an aqueous coating composition and a powder clear coating composition are used have been unsuccessful.
Thus, in Japanese Kokai Publication Sho-52-069446, application of an aqueous base coating composition is followed by preheating under very particular conditions so that the volatile matter content in the paint film can become 6% or lower. Then, a powder coating composition is applied and baking is performed.
In view of the foregoing, the advent of a two coat one bake method capable of giving coatings excellent in appearance by applying an aqueous base coating composition and then, without curing the base, applying a powder clear coating composition, followed by baking is awaited. It is an object of the present invention to meet such demand.